Mac gets gradually slower until it's almost unusable

A couple of months ago something got corrupted on my hard drive which caused the computer to not boot properly. I was able to back it up, repair the drive, and reinstall the OS.

Everything was fine, but things seemed to be slower, though that could just be in my head.

Fast forward to now... I always leave my computer on. Yeah. I hate the environment. I came in one day and my computer had powered down sometime during the night, so I rebooted it and noticed that my battery no longer charges. I keep my computer plugged in, so not sure if the battery issue would have powered down my computer in and of itself. It shouldn't have.

Then I noticed slowness. Like unbearable slowness - just using Office and surfing the web. A lot of pages become unresponsive and/or take forever to load. It's not my internet connection. I was able to finally get a speedtest to load and speed was fine (25mbps down and 20 up). Also, I have a bunch of other computers, including an Air and they were fine on the same network. Also, EVERYTHING becomes slow, not just my browser. Even System Preferences and Finder take forever to open (spinning beachball).

It's not RAM, diskspace or CPU, based on looking at my activity monitor, so the only think I can think is perhaps my hard drive is failing.

If I close everything and reboot, then speed will resume back to normal, but then it will start to slow again after a few hours of usage. I guess the other thing it could be is one of the programs I run. I generally run Chrome, Outlook, word, excel, powerpoint, itunes (streaming via icloud), Lync and Skype. I supposed it could be one of those programs, but nothing in Activity Monitor shows any high usage by any one process.

I have an appointment to take my computer in, but my fear is that it'll be running fine. I have to be doing things for a few hours before it becomes super super slow. It almost seems like a memory leak somewhere.

I've run disk utility and it can't find anything wrong with the hard drive. The drive isn't making any strange noises or clicking. My computer does take about a minute to boot up these days though. I don't recall it being that sluggish before, but maybe it was.

Any ideas on what could be the problem or recommendations for other things to try? I was unable to successfully do a Hardware Test. I tried a zillion times (booting up while pressing "D") and nada.

Macbook pro 15" Early2011

8G RAM

2.3 ghz i7 core

OSX 10.8.4